# How to compute the TOD (Top of Descent) - Thumb rule

#31

Okay, thanks a lot.

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#32

regards cruising speed this is usually about 300-310 kts depening on the aircraft. Above Fl280 this is M0.76-M.082

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#33

Okay, thanks a lot!

#34

There’s a little mistake…there is a zero too much when you devide 20000 by 1000(you wrote 10000) ;-)
You wrote 20000-10000=20
but it has to be 20000-1000=20
;-)

…thanks anyway for the tutorial

Is there any tutorial “how high to climb” /“how to chose correctly the right cruising altitude depending on the distance between 2 airports”;-)

/Ronny

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#35

There’s a post somewhere about that, but I don’t remember where.

Here’s what I’ve written down from that post:

Take your total route distance, divide by 2, subtract 10, divide by 3 and multiply by 1000.

Ex: 300NM / 2 = 150 - 10 = 140 / 3 = 46.666 x 1000 = 46,666 ft. (obviously unrealistic to fly that high, but use it as a reference to how high you can go in order to descendin time.)

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#36

I found the comment! Hope this helps. :)

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#37

Thanks!;-)

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#38

What I do is your current altidute (eg. 30,000) minus your target altitude (eg. 125) divided by 0.0524 (tan3°) That number is in feet so turn it into miles! This is an exact number… Probably the ones above are a lot easier.

#39

Thanks for posting! I had run across this a couple of times in various places but now couldn’t remember the rule nor where I’d read it.

While very clear and just for the sake of clarity for others, I believe there is a typo in step 2 of the example, where 20,000 would be divided by 1,000, not 10,000 (I.e. 20000 / 1000), right?

Thanks again!

#40

Too make things easy I use the FL devided by 3 as this gives you the answer without need for further maths!

Eg FL320 / 3 = 106 nm to go.

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#41

I use a more complicated one. (ALT - (Field Elevation+2500)) *3=X/1000+10
Basically you take your altitude and minus that with the Field Elevation of your arrival airport, plus 2500, which is the intercept altitude. Then you multiply the result from the earlier calculation (ALT-(FLD ELEV+2500)) with 3. That gives X. The result for that is X. But since your result is then in the high thousands, and in case you are too lazy to remove three "0"s, you divide by 1000. The number you get then you plus with 10, as you want to intercept the localizer 10nm out.

Now, lets say you´re cruising at 35000ft, and Approaching KATL. KATL has a field elevation of about 1000ft.
So the calculation will be: (35000 - (1000+2500)) *3=94500/1000=94,5+10=104.5. Your Top of Descend will then be 104.5 nautical miles out of the airport.

Hope it was not too complicated. :)

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#42

I’ll start using this formula instead. It is definitely more accurate. Thanks for sharing. :)

#43

What is the difference between multiplying by three dividing by 1000 and simply dividing by 3

#44

When you say miles you mean nautical miles or we convert miles to nm?

#45

Or just use the new IF Checklists calculator😉

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#46

Hey,
I very much apologise if someone has already covered but I have to questions to your rule of thumb. Firstly why is the target at 10,000ft and not like 3,500ft to pick up the ILS ect? And secondly you have established 60 miles but you haven’t given an exact rate of descent which got me thinking. If I descended two (two separate flights from the same place) wouldn’t I be at a different place if I descended at 2000ft/min than descending at 3000ft/min or more?

Thanks for any responses even if they are that this has been covered and I apologies if that is the case.
Thanks, James L

#47

When i do long flights, cruising altitude 40,000ft. At 300nm from destination, I start descending for 30,000ft at vertical speed of 1000. Then 200nm-20,000ft, But at 120nm I start to reduce my speed to 220knots. Then 100nm-10,000 same vertical speed. Just before I reach 50nm reduce speed to 180knots AP off. Using the picture below as my altitude guide before reaching ILS (if there is no approach tower to guide you).

#48

This video by a professional pilot, Captain Joe, shows how it’s done. Interesting he also uses the FL/3 technique as well. ( however adds in some for the slow down etc)

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Phases of descent and approach
#49

I use a flight computer, it also helps me in ground school and I can use it when I go into take my FAA Knowledge exam 😁

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#50

Is that an App or is it a physical computer?

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